Voltage surge arresters currently employed as station arresters and distribution arresters generally consist of the series combination of a plurality of spark gap devices and variable resistance devices in series. The series combination of the spark gap and varistors are generally enclosed within a hermetically sealed insulating container having one end connected to line and another end connected to ground. Upon the occurrence of a voltage surge the resulting voltage across the arrester causes the spark gaps to spark over and the varistors to become conductive. After the cessation of the overvoltage surge condition the spark gaps clear the power follow current and an open circuit condition results across the arrester terminals.
Surge voltage arresters employ a silicon carbide (SiC) material in cylindrical form as a variable resistance element and a plurality of apertured aluminum oxide discs with electrodes for providing the series spark gap stucture. The exponent n, which determines the change of resistance with voltage for varistor devices, is relatively low for SiC varistors. Since the surge arresters are continuously coupled between line and ground the spark gap structure insures that continuous current does not flow through the SiC varistor device. Continuous flow of current through a SiC varistor under steady state conditions would cause the varistor to become conductive in the absence of an overvoltage condition.
Also considered for use within voltage surge arresters are zinc oxide (ZnO) varistor devices having a high n value. Since the exponent values for ZnO varistors are substantially higher than for SiC varistors, ZnO units can provide more protection than the SiC varistors. Since ZnO varistors are not used with series gap elements continuous varistor current flows to ground causing substantial varistor heating. In order to prevent the thermal instability associated with this steady state heating the ZnO varistors are encapsulated within a heat sink and heat transfer medium to keep the ZnO material at low operating temperatures and currents. The substantial quantity of encapsulant required renders ZnO varistors infeasible for compact surge arrester applications where size is of the essence.
One patent, U.S. Pat. No. 3,778,743 issued Dec. 11, 1973 discloses the combination of a plurality of zinc oxide varistors in series with at least one spark gap for use in a lightning surge arrester. The varistors are disclosed as substitutes for silicon carbide varistors. The direct replacement of zinc oxide varistors for gapped silicon carbide varistors is not economically feasible at this time due to the greater expense involved in the materials and manufacture of zinc oxide varistors and the expense involved in the materials used within state of the art gap devices. Also disclosed within the same U.S. patent is the use of zinc oxide varistors without spark gaps but no mention is made of the thermal instability that occurs when no heat sinking is provided. However, for humidity purposes, the varistors are disclosed as embedded in epoxy and phenol resin.
The purpose of this invention is to describe a compact surge arrester employing ZnO varistors in series with miniaturized series gap devices of special construction.